The present invention relates to a liquid crystal device for use in flat-panel displays, projection displays, printers, etc., and a driving method for the liquid crystal device.
Heretofore, various liquid crystal devices (panels) utilizing a liquid crystal for effecting optical switching have been proposed.
As a type of a nematic liquid crystal display device, there has been known an active matrix-type liquid crystal device wherein each pixel is provided with an active element (e.g., a thin film transistor (TFT)).
As a nematic liquid crystal material used for such an active matrix-type liquid crystal device using a TFT, there has been presently widely used a twisted nematic (TN) liquid crystal as disclosed by M. Schadt and W. Helfrich, xe2x80x9cApplied Physics Lettersxe2x80x9d, Vol. 18, No. 4 (Feb. 17, 1971), pp. 127-128.
In recent years, there has been proposed a liquid crystal device of In-Plain Switching mode utilizing an electric field applied in a longitudinal direction of the device, thus improving a viewing angle characteristic compared with TN-mode liquid crystal displays. Further, a liquid crystal device of a super twisted nematic (STN) mode without using the active element (TFT etc.) has also been proposed.
In any mode however, the resultant nematic liquid crystal display device has encountered a problem of a slow response time of several ten milliseconds or above.
In order to solve the above-mentioned difficulties of the conventional types of nematic liquid crystal devices, a liquid crystal device using a liquid crystal exhibiting bistability (xe2x80x9cSSFLCxe2x80x9d, Surface Stabilized FLC), has been proposed by Clark and Lagerwall (Japanese Laid-Open Patent Application (JP-A) 56-107216, U.S. Pat. No. 4,367,924). As the liquid crystal exhibiting bistability, a chiral smectic liquid crystal or a ferroelectric liquid crystal (FLC) having chiral smectic C phase (SmC*) is generally used. Such a chiral smectic (ferroelectric) liquid crystal has a very quick response speed because it causes inversion switching of liquid crystal molecules by the action of an applied electric field on spontaneous polarizations of their liquid crystal molecules. In addition, the chiral smectic liquid crystal develops bistable states showing a memory characteristic and further has an excellent viewing angle characteristic. Accordingly, the chiral smectic liquid crystal is considered to be suitable for constituting a display device or a light valve of a high speed, a high resolution and a large area.
In recent years, as another liquid crystal material, an antiferroelectric liquid crystal showing tristability (tristable states) has caught attention. Similarly as in the ferroelectric liquid crystal, the antiferroelectric liquid crystal causes molecular inversion switching based on the action of an applied electric field on its spontaneous polarization, thus providing a very high-speed responsiveness. This type of the liquid crystal material has a molecular alignment (orientation) structure wherein liquid crystal molecules cancel or counterbalance their spontaneous polarizations each other under no electric field application, thus having no spontaneous polarization in the absence of the electric field.
In the case of using the chiral smectic liquid crystals (ferroelectric and antiferroelectric liquid crystals), similarly as in the case of the above-mentioned nematic liquid crystals, a larger contrast is required of resultant display images.
Further, in the case of the above-mentioned device (cell) using the chiral smectic liquid crystal, it has been difficult to effect a gradation display due to its display principle utilizing bistability.
In recent years, in order to allow a mode of controlling various gradation levels, there have been proposed liquid crystal devices using a specific chiral smectic liquid crystal, such as a ferroelectric liquid crystal of a short pitch-type, a polymer-stabilized ferroelectric liquid crystal or a threshold-less anti-ferroelectric liquid crystal. However, these devices have not been put into practical use sufficiently.
On the other hand, with respect to motion picture image qualities, it has been clarified that the motion picture image qualities are improved by providing a non-image display period between a still image display period and a subsequent still image display period when the above-mentioned conventional liquid crystal device is used for displaying motion picture images by sequentially rewriting the still images (a described in, e.g., xe2x80x9cShingaku Gihoxe2x80x9d (Technical Report of IEICD), EID 96-4 (1996), p. 16).
However, in the conventional nematic (display) mode, the response speed of a liquid crystal is insufficient, thus failing to be applied to the above motion picture display schemes. Further, in the case of using the chiral smectic liquid crystal (such as, a ferroelectric liquid crystal of a short pitch-type, a polymer-stabilized type or a threshold-less antiferroelectric liquid crystal), the resultant liquid crystal device is accompanied with difficulties, such as complicated driving method and peripheral circuits, thus leading to an increase in production cost.
In view of the above-mentioned problems, an object of the present invention is to provide a liquid crystal device capable of preventing a lowering in contrast while keeping a good gradation display performance.
Another object of the present invention is to provide a method for driving the liquid crystal device.
According to the present invention, there is provided a liquid crystal device, comprising:
a chiral smectic liquid crystal, and a pair of substrates having thereon electrodes for applying a voltage to the chiral smectic liquid crystal and disposed to sandwich the chiral smectic liquid crystal to form a plurality of pixels, wherein
the chiral smectic liquid crystal has a temperature-dependent tilt angle characteristic satisfying the following relationship:
Ĥ10xe2x88x92Ĥ1xe2x89xa64.0 degrees,
wherein Ĥ10 denotes a tilt angle at a temperature which is 10xc2x0 C. lower than an upper limit temperature of chiral smectic C phase and Ĥ1 denotes a tilt angle at a temperature which is 1xc2x0 C. lower than the upper limit temperature, and
the liquid crystal has an alignment characteristic such that the liquid crystal is aligned to provide an average molecular axis to be placed in a monostable alignment state under no voltage application, is tilted from the monostable alignment state in one direction when supplied with a voltage of a first polarity at a tilting angle which varies depending on magnitude of the supplied voltage, and is tilted from the monostable alignment state in the other direction when supplied with a voltage of a second polarity opposite to the first polarity at a tilting angle, the tilting angles providing maximum tilting angles formed under application of the voltages of the first and second polarities, respectively, different from each other,and one of the tilting angles continuously changing depending on magnitude of the supplied voltage of the first polarity or the second polarity.
According to the present invention, there is also provided a method of driving a liquid crystal device, comprising:
applying a voltage corresponding to a gradational display level to the chiral smectic liquid crystal.
According to the present invention, there is further provided a method of driving a liquid crystal device, comprising:
driving one frame period into a plurality of field periods,
displaying a higher luminance image in at least one field period and a lower luminance image in at least one field period.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.